1. Field of the Invention
The invention relates to the field of digital audio recording and, more specifically, to a method and apparatus for recording multiple tracks of digital audio information onto magnetic tape using a rotary magnetic head, with high dropout immunity, reduced susceptibility to head misalignment and improved ability to scan at high linear tape speeds.
2. Description of the Prior Art
A full fidelity digital audio recorder requires a 1 MHz storage bandwidth per recorded audio channel. Because a rotary head digital recorder has a wide bandwidth, it is an ideal storage device for digital audio signals. Using digital procesing techniques, an audio signal is converted into a digital format and recorded onto a magnetic medium. Upon playback, the digital signal is converted first to a corrected digital audio bit stream and then to an analog waveform.
The high bandwidth required to effectively record a digital audio signal necessitates a high tape speed. Rotary head recorders achieve a high bandwidth as a result of high relative heat-to-tape speed. Media recorded using a rotary head can achieve 60,000 flux transitions/inch, about the same as stationary head recording, but track density may reach 2,000 tracks/inch, or about 10 times that of stationary head recording. Data transfer rates of 1 to 4 Mbytes/second are obtained, as the rotary head records the digital audio data onto the tape using a technique called helical scanning. In helical scanning, two or more electromagnetic heads are positioned on a head cylinder. The head cylinder is rotated at a speed (e.g., 3,000 revolutions/minute) that allows the use of a lower tape speed while still maintaining a high relative head-to-tape speed. As shown in FIG. 1, the tape is helically wrapped around a portion of the cylindrical drum.
Helical scanning records diagonally aligned tracks onto the tape. Because the tape is guided past the heads at an angle, each recorded track is placed diagonally across the tape width. FIG. 2 illustrates the diagonal tracks created by helical scanning. Each of the two heads on the cylinder alternately lays down a track 201 onto the tape 202. Each pair of tracks 203 represents a specified amount of audio signal. The tracks are recorded side by side without any space separation.
When re-recording a signal onto digital tape, the new signal is recorded over the signal previously recorded on the tape. The accumulation of debris on the tape can cause a media dropout, or recording omission, so that the new data is not properly recorded and old data is therefore not completely erased. After the re-recording operation is performed, and the debris is removed from the surface of the magnetic tape, the read head may detect old data on the track at the point where the dropout occurred. It is difficult to detect, using standard detection means, such an erasing or recording omission. When the new data is reproduced along with the old data in a digital audio system, abnormal sounds may be generated due to the unerased old data.
In professional digital audio applications, in addition to being able to record several channels on a single tape, it is often desirable to be able to selectively re-record individual channels. For example, a guitarist, after reviewing a studio session recorded with other musicians, may want to re-record a portion of his guitar solo, while leaving the rest of the recording unaltered. Additionally, possibly because he is no longer at the original studio, the guitarist may want to re-record using a different digital audio recorder than was originally used. Selectively re-recording a portion of a single channel (called "punching-in") is difficult because of the high density of data recorded using a rotary head, and because some dissimilarities in head alignment may exist between the two machines. If the new guitar solo is not precisely re-recorded over every group of bits of the old solo on the tape, the digial audio system is not able to differentiate between the old and new data on the tape. Consequently, during playback, the listener will hear portions of the old solo mixed with portions of the new solo.
During normal operation, the rotary head detects all of the information stored on the magnetic tape. However, in a high-speed scan, or "shuttle" mode, the linear tape speed is increased while the rotational speed of the rotary head is kept constant. It is desirable to record the digital information onto the magnetic tape so that the digital audio system can accurately locate and monitor specific audio portions of the tape during both normal and shuttle operation. For example, such a tape format is particularly suited to synchronizing applications, where one or more slave recorders are synchronized with a master recorder, and the slave and master tapes are operated with a precise timing relationship to one another at both normal operating speeds and higher scanning speeds. For an example of one possible synchronizing system, see the invention described in co-pending U.S. application Ser. No. 07/822,464, entitled "Method For Synchronizing Digital Audio Tape Recorders," filed Jan. 17, 1992, and assigned to the Assignee of the present invention.
Disadvantages of rotary head designs arise mainly in professional applications. Razor blade editing is not possible with a rotary head data track as it is with a stationary head recorder. Therefore, rotary head designs require electronic editing. Expensive and complex digital audio editors are required to perform electronic editing in the prior art. Another disadvantage of rotary head design stems from the multiplexing of channels: it is difficult to record and replay separate channels simultaneously. Similarly, punch-in/out is not readily feasible. Punch in/out describes a procedure whereby as a channel is played back it is placed in the record mode at a certain point in the music to record new material, then taken out of the record mode at the end of the new passage to be recorded.
Rotary head designs are used for some 2-track professional applications. Nevertheless, professional audio applications that require electronic editing and multi-track and synchronous recording capabilities have typically retained the stationary head design for digital audio recording.
One rotary head transport system optimized for digital audio is the Digital Audio Tape (DAT) cassette format. The standard DAT system uses tape that is 3.81 mm wide capable of recording up to two hours of 2 channel digital audio onto a single cassette. The DAT system also supports 4 channel recording of 12-bit nonlinear quantized audio data using a 32 kHz sampling rate.
One prior art digital audio rotary head system is capable of recording 12 tracks of digital audio onto a specially designed 8 mm cassette. Because of the relatively small tape width, however, the prior art system's high bit density renders it highly susceptible to errors due to media dropouts. Further, the system is only capable of recording approximately 15 minutes of digital audio information per cassette.
Tape cassettes that contain 8 mm wide tape are of fairly good quality and are generally available, but cannot accommodate many tracks without a significant decrease in total playing time. The same holds true for the 3.81 mm R-DAT tape. One-half inch wide tape is superior in that it is widely available and can accommodate multiple channels of digital audio without sacrificing playing time.
The use of a rotary head design for digital audio recorders offers the opportunity to achieve the high bandwidth required for digital audio storage. The narrow track width and high tape-to-head speed of rotary head designs yield high recording density and low tape consumption. A digital audio tape format for use with one-half inch tape is an attractive alternative to prior art systems that use 8 mm or R-DAT (3.81 mm) tape. One-half inch tape is wider, allowing more data to be recorded per linear inch without increasing dropout susceptibility. Further, standard one-half inch cassettes provide longer playing times than standard 8 mm or R-DAT tapes. Also, mass production of one-half inch tape allows it to be manufactured at low cost, making a relatively sophisticated storage medium available to both consumers and professionals. It is therefore desirable to design a tape format that can be used to record multiple channels of digital audio information on one-half inch tape.